Introduction
Chlorite is a general name for a group of magnesium rich hydrous sheet silicates possessing similar structure and chemical composition. Many green rocks owe their color to chlorite and chlorite owes its name to its green color. The Greek word choros, meaning “green,” is the origin of chlorite’s name. Consisting of negatively charged mica-like (2:1) layers regularly alternating with positively charged brucite-like (octahedral) sheets, the basic structure of chlorites allows for various compositions(Grim,1962). Members are differentiated by substitutions within the octahedral layer and the tetrahedral or octahedral positions of the mica-like layer. The arrangement of hydroxide and 2:1 sheets stacked in the z-direction defines the polytype of chlorite. There are theoretically six possible layer interlayer assemblages in either semi-random or regular "one-layer" polytypes. Of these six polytypes, four have been observed in nature (Partice De Caritat ect. 1993): clinochlore (Mg,Fe2+)5Al(AlSi3O10)(OH)8, chamosite (Fe2+,Mg)5Al(AlSi3O10)(OH)8, nimite (Ni,Mg,Al)6((Si,Al)4O10)(OH)8, and pennanite Mn52+Al(AlSi3O10)(OH)8.
Provenance
Chlorite is widespread throughout the world, often found in low- to medium-grade regional metamorphic rocks and as a secondary mineral to mafic silicates in igneous, metamorphic, and sedimentary rocks. It is an occasional constituent of igneous rocks, in most cases probably forming secondarily by deuteric or hydrothermal alteration of primary ferromagnesian minerals, such as mica, pyroxene, amphibole, garnet, and olivine (Bailey, S. W., 1988). A ubiquitous product of low-grade metamorphism, chlorite influences a wide variety of rocks of different formation age such as in Cambrian basaltic vesicular lavas, early Oligocene sandstones and greywackes with andesitic volcanic clasts (Lopez-Munguira et al., 2002) (Schmidt and Levi, 1999). Chlorites are also common constituents of argillaceous sedimentary rocks where these minerals occur in both detrital and authigenic forms. A massive compact variety of the chlorite group, clinochlore is used as a decorative carving stone is referred to by the trade name seraphinite. It occurs in the Korshunovskoye iron skarn deposit in the Irkutskaya Oblast of Eastern Siberia (Mazurov, M.P., Grishina, S.N., Istomin, V.E., and Titov, A.T., 2007). Recent evidence shows some chlorite forms during diagenesis in nearshore marine sediments. Chlorite is common in soils although it is normally a minor component.
Physical Properties
Individual species and varieties of chlorite are very difficult to distinguish with only hand specimens. Micaceous cleavage, crystal habit, hardness, and green color are usually reliable for hand identification. Chlorite has a hardness of 2-2.5, one perfect basal cleavage at (001), flexible fracture, and habits similar to the other micas. Foliated books, scaly aggregates, and individual flakes in a quartz-feldspar matrix are common, rare pseudohexagonal crystals are known (Perkins, 2011). Pseudohexagonal platelets paralleI to the basaI pinacoid (001) occur in the best crystallized varieties. These plates range in width from less than a millimeter up to several inches. It has a vitreous luster and appears transparent to translucent, with variable green color. The density varies between 2.6 and 3.3, depending on composition (Bailey, S. W., 1988). It has a white to colorless streak chlorite is sometimes confused with talc.
Composition
Chlorites are hydrous aluminosilicates of complex chemical composition and structure. The formula for chlorite is (A) 5-6(T) 4 (Z) 18 where A = Al, Fe2+, Fe3+, Li, Mg, Mn, or Ni, while T= Al, Fe3+, Si, or a combination of them, and (Z) = O and/or OH. Chlorite incorporates primarily Mg, AI and Fe, and to lesser extent Cr, Ni, Mn, V, Cu, and Li in the octahedral sheet within the 2: 1 layer and in the interlayer hydroxide sheet. Chlorites exhibit a large substitution of Si by Al. All chlorites consist of alternating talc-like and brucite-like layers with variable stacking orders in both layers. In both talc-like and brucite-like layers, iron and aluminum may substitute for magnesium, while other elements such as nickel may be present. The tetrahedral portions of each 2: 1 layer have negative charges due to ionic substitution of AI3+, or occasionally of Fe3+ or Cr3+, for Si4+. Interlayer sheets have a positive charges due to substitution of AI3+, Fe3+, and Cr3+ for Mg2+, Fe2+, Mn2+, and Ni2+ and, neutralize the negative charges on the 2: 1 silicate layers. It is usually impossible to determine if the tetrahedral charge is compensated completely within the interlayer sheet or whether the octahedral portion of the 2: 1 layer also acquire a positive charge. The main components of the two octahedral sheets are Mg, Fe2 +, Al, and Fe3 + with substitutions of Cr, Ni, Mn, V, Cu, or Li in specific varieties. The absolute quantities of the elements present in the unit cell can be obtained if the density and the volume of the unit cell are available in addition to the chemical analysis. In most cases, this is not possible, and the allocation to a structural formula must be made on the basis of some assumption about the formula unit (Bailey, S. W., 1988). Chlorite structure and chemical composition can reflect the physiochemical conditions of its formation. The Fe¬2+/ (Fe2++ Mg) ratio differs for chlorites formed in different in hydrothermal environments, showing that the iron content in chlorite is temperature related (Ciesielczuk J., 2012). Pictured below is a chlorite group mineral. Optical Properties
When observed in thin section, chlorite is light to medium green with pleochroism expressed in shades of green. Darker varieties are Iron-rich. The relief index for chlorite is moderate to moderately high positive with nα=1.55-1.67, nβ=1.55-1.67, and nδ=1.55-1.69. Birefringence is 0.0-0.015 with first order white to yellow, chlorite may exhibit anomalous blue or purplish interference colors. Alteration by oxidation may produce iron stains, but chlorite is relatively stable. Chlorite crystals are nearly isotropic for most specimens, and the optic sign changes from negative on the Fe-rich side to positive on the Mg-rich side. Abnormal interference colors are blue near the sign change due to the mineral becoming isotropic for part of the spectrum and anisotropic for another part. The abnormal colors are blue to violet on the Fe-rich side and brown on the Mg-rich side. The birefringence increases in both directions away from the sign change (Bailey, S. W., 1988). The distinguishing optical features of chlorite are green color, weak pleochroism, and weak birefringence. Below are high resolution pictures of thin section chlorite in schist.

Chlorite in Schist. Digital images. Chlorite. N.p., n.d. Web. 1 May 2015.
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Crystallography
Chlorite is mainly monoclinic, while triclinic or orthorhombic varieties are less common. Its morphology is very diverse. Pseudohexagonal platelets parallel to the basaI pinacoid (001) occur in the best crystallized varieties. These plates range in width from less than a millimeter up to several inches (Bailey, S. W., 1988). It belongs to the space group C2/m, and point group 2/m. The pronounced pseudohexagonal geometry is due to the hexagonal patterns of the tetrahedral and octahedral sheets. The presence of layers not held together by anion-cation contacts explain their excellent cleavage along (001). The cell parameters of chlorite are a = 5.37, b = 9.30, c = 14.25, β = 97.4°, Z = 2. Chlorites have their 001 peaks at 14 to 14.4 angstroms, depending on the individual species. Peak positions are unchanged by ion saturation, solvation with ethylene glycol, or heating. However, heat treatments above 500 C alter peak intensities (Barnhisel and Bertsch, 1989). Chlorites are not as soft as kaolins or talc because the 2: 1 and interlayer units are charged alternately negative and positive.

Atomic Structure
Chlorite group minerals have a 2:1 sandwich structure of two tetrahedral sheets and one octahedral sheet are combined to form a 2: 1 unit similar to that in mica, but excluding the interlayer K found in true mica. Space between each 2:1 sandwich is filled by a cation composed of (Mg2+, Fe3+)(OH)6, known as the brucite-like layer. Chlorites as they occur in nature are layer lattice silicates of one of two polymorphic types. Typically one is more perfect and coarsely crystalline, and corresponds with the mica scheme of crystallization. It is a four-layer structure consisting of micaceous sheets and substituted brucite-like sheets. The other is more fine grained, less perfectly crystallized, and has a two-layer trioctahedral structure. The probable number of different two-layer chlorite structures is 1134. Of these, 1009 have monoclinic-shaped unit cells, and 125 have orthorhombic-shaped cells (Bailey, S. W., 1988). In chlorites atomic structure, layers of the same type can be superimposed to form twelve unique 1-layer polytypes having a regular stacking sequence and six structures having a semirandom stacking sequence. There are four possible ways of positioning the brucite-like sheet on the initial talc-like sheet and six positions that may be assumed by the repeating talc-like sheet. Certain structures are equivalent to others after being rotated 180° about the y axis or because of an enantiomorphic relationship consequently, only four of the six positions for the repeating talc-like sheet need to be considered. In two-layer polytypes equivalences in individual 1-layer structures must be disregarded because each 1-layer unit is no longer unique but is part of a 2-layer structure. Thus all six possible positions of the repeating talc-like sheet must be considered. To complicate further the derivation of the 2-layer structures, the orientation of the talc-like sheet need not be identical in each successive layer as is true for regular 1-layer structures, and shifts within the talc-like sheet itself also need to be considered (Lister J. Bailey S.W.,1967).
Article Summaries
Chlorite Polytype Geothermometry, Jeffery R. Walker, Department of Geology and Geography, Vassar College, Poughkeepsie, New York 12601, Clays and Clay Minerals, Vol. 41, No. 2, 260-267, 1993.
Stability of various chlorite polytypes may be a function of the temperature at which they formed. Upon reviewing the reported occurrences of chlorite, other factors, such as grain size of the parent rock may be at least as important as temperature in controlling the stabilities of each polytype. The study focuses on the structural details of polytype transformations on the relationship of polytype stability to pressure, composition and kinetics, and on experimental calibration of the transformations. The transitions between polytypes probably involve dissolution and recrystallization because of rotations and or translations of various elements of chlorites structure.
Determination of Chlorite Cmpositions by X-Ray Spacings and Intensities. S.W. Bailey, Department of Geology and Geophysics (1972) University of Wisconsin, Madison, Clays and Clay Minerals, 1972.Vol. 20, page(s) 381-388
X-ray spacing has been cited as empirical measures of the tetrahedral and octahedral cation population of chlorite minerals. The cell dimensions and compositions of four chlorites whose crystal structures have been determined are used to test existing graphs and regression equations. Based on comparison with the four test chlorites, both tetrahedral Al and total octahedral heavy atoms can be estimated by X-ray spacing methods with an average error of about 10%.
Chlorite crystallinity as an indicator of metamorphic grade of low-temperature meta-igneous rocks: a case study from the Btikk Mountains, Northeast Hungary, P. Arkai and D. Sadek Ghabrial. Laboratory for Geochemical Research, Hungarian Academy of Sciences, Budapest, Hungary (Received 15 January 1996; revised 19 June 1996) Clay Minerals (1997) Vol. 32 page(s) 205-222
For meta-sedimentary rocks, chlorite crystallinity can be used as an empirical tool to determine relative differences in grades of low-temperature meta-igneous rocks. Electron microprobe and XRD data show that chlorite crystallinity is controlled mainly by the decreasing amounts of contaminants in chlorite with advancing metamorphic grade. This may be used for estimating the approximate temperatures of metamorphism, in spite of the contrasting interpretation of chemical data obtained by electron microscope analyses.
Chlorite of hydrothermal origin in the Strzelin and Borów granites, Justyna Ciesielczuk, Fore-Sudetic Block, Poland, Geological Quarterly, 2012, 56 (2): 333–344
Chlorite is used as a geothermometer due to its structure and chemical composition which reflect the physical and chemical conditions of its formation. Hydorthermally altered Strzelin and Borow granites have two forms of chlorite, spherulitic and post-biotote. They formed differently as a result of the replacement of biotite or crystallization from fluid. Temperature range of chlorite formation corresponds to the zeolitic-prehnite/pumpellyite facies of low-grade metamorphism. Calculated temperatures do not take into consideration the pressure, oxygen fugacity, activity of Mg2+ ions, sulphur concentration, pH or ionic concentration.
Weathering of chlorite to a low-charge expandable mineral in a spodosol on the Apennine Mountains, Italy, Stefano Carnicelli, Aldo Mirabella, Guia Cecchini and Guido Sanesi, Clays and Clay Minerals, Vol. 45, No. 1, page(s) 28-41, 1997.
The first stage of weathering in chlorite happens in the lower solum, involving iron oxidation, partial removal of originally magnesium-rich hydroxide sheet, and deposition of aluminum in the partially accessible interlayer space. In the OH-stretching region, it is possible to observe in the BC horizon the presence of an absorption band. The shift to higher wave numbers of the main interlayer OH-stretching band in the infrared spectra shows evidence that iron is preferentially removed from the interlayer sheet. The findings support possibility of transformation of trioctahedral ferruginous chlorite into dioctahedral smecite in Spodosols.
Chlorite dissolution in the acid pH-range: a combined microscopic and macroscopic approach, Felix Brandt, Dirk Bosbach, Evelyn Krawczyk-Bärsch, Thuro Arnold, Gert Bernhard, Geochimica et Cosmochimica Acta, Volume 67, Issue 8, 15 April 2003, pages 1451–1461
In some studies, chlorite dissolution in the acid pH range is reported to be incongruent. The reactive surface area of dissolution is a critical parameter of the reaction. More recent studies demonstrated the dissolution of sheet silicates is limited to small parts of the total surface. These studies’ results propose two mechanisms that control chlorite dissolution at low pH: defect controlled dissolution of molecular steps on basal surfaces, and transformation of parts of the chlorite structure into interstratified vermiculite/smectite.
Crystallochemical classifications of phyllosilicates based on the unified system of projection of chemical composition: ii. The chlorite group, A. Wiewiora and Z. Weiss, Clay Minerals (1990) Vol. 25 page(s) 83-92.
A new classification based on the unified projection system of chemical composition is proposed in which all trioctahedral, di-trioctahedral and dioctahedral species of chlorite are incorporated. A new superior formula for trioctahedral and dioctahedral chlorites is created as end member compositions leave many compositions unnamed.
Nomenclature of the Trioctahedral chlorites. Peter Bayliss, Canadian Mineralogist (1975) Vol. 13, page(s) 178-180
Trioctahedral mineral species within the chlorite group have the end-member oornpositions as follows: clinochlore (MgsAlXSir,Al)Oro(OH)e, chamosite, nimite, and pentrantite. The varietal names of brunsvigite, corundophilite, daphnite, delessite, diabantite, grovesite, kiimmererite, kotchubeite, leuchtenbergite, orthochamosite, pennine, pseudothuringite, pycnochlorite, ripidolite, sheridanite, talc-chlorite and thuringite should be discarded. There are descriptions at present for four distinct chemical species within the trioctahedral chlorite group. With the use of chemical element adjectives, the complete range of chemical varieties may be described, so that all special varietal names should be discarded. In addition, the polytype symbol should be stated if it can be determined.
Synthesis of the chlorites and their structural and chemical constitution. Bruce W. Nelson and Rustum Roy, The American Mineralogist (1958) Vol. 43, July-August.
A satisfactory systemization of the chlorite group is very difficult because the discrepancies between chemical and structural assemblages and the fact that data of this nature was nearly impossible to come by in the late 50’s. Structural information revealed that most chlorites contain alternating mica-type and brucite-type layers in their crystal structures, while others are represented by layer structures containing trioctahedral units. The existence of these two polymorphic isostructural series supplies a significant advance in understanding of the chlorites at the time. This paper discussed the magnesian chlorites.
Origin and impact of authigenic chlorite in the Upper Cretaceous sandstone reservoirs of the Santos Basin, eastern Brazil, Andréa B. Bahlis and Luiz F. De Ros (2013) Petroleum Geoscience, Vol. 19 page(s) 185-199
Santos sandstones are mostly fine-grained, lithic arkoses, rich in volcanic rock fragments (VRF). Chlorite, the most abundant diagenetic constituent, occurs as coatings, rims, rosettes and replacing grains. Chlorite precipitation was favored by the presence of eodiagenetic smectite coatings and by the abundance of VRF. Detrital heavy minerals, biotite and mud intraclasts were also sources and/or substrates for chlorite authigenesis.

References Cited
Bailey S. W. (1988): Chlorites: Structures and crystal chemistry. Reviews in Mineralogy 19, 347-403.
Barnhisel, R.I. and P.M. Bertsch. (1989) – Chlorites and Hydroxy-Interlayered Vermiculite and Semecite, J.B. Dixon S.B. Weed (eds.). Madison, Wisconson, Mineral in soil enviroments. Soil Science Society on America
Chlorite in Schist. Digital image. Chlorite. N.p., n.d. Web. 1 May 2015. .
Chlorite Atomic Structure. Digital image. U. S. Geological Survey Open-File Report 01-041. Coastal and Marine Geology Program, n.d. Web. 1 May 2015. .
Chlorite Structure Crystal Shape. Digital image. U. S. Geological Survey Open-File Report 01-041. Coastal and Marine Geology Program, n.d. Web. 1 May 2015. .
Ciesielczuk J. (2012) – Chlorite of hydrothermal origin in the Strzelin and Borów granites (Fore-Sudetic Block, Poland). Geol. Quart., 56 (2): 333–344
Grim R.E. (1962) Applied Clay Mineralogy, McGraw Hill, New York, NY
Lister J. Bailey S.W. (1967) - Chlorite Polytypism: IV. Regular Two-Layer Structures. Department of Geology, University of Wisconsin, Madison Wisconsin. The American Mineralogist, Vol. 52, November-December
Lopez-Munguira A., Neito F. and Morata D. (2002) – Chlorite composition and geothermometry: a comparative study of Cambrian basic lavas from the Ossa Morena Zone, SW Spain. Clay Miner., 37: 267–281
Mazurov, M.P., Grishina, S.N., Istomin, V.E., and Titov, A.T. (2007): Metasomatism and Ore Formation at Contacts of Dolerite with Saliferous Rocks in the Sedimentary Cover of the Southern Siberian Platform. Geology of Ore Deposits 49(4), 271-284.
Partice De Caritat, Ian Hutcheon, and John L. Walshe (1993) – Chlorite Geothermometry: A Review (Aberta, Canada). Clays and Clay Minerals, Vol. 41 No.2 219-239
Perkins, Dexter (2011), Mineralogy 3rd Edition, Prentice Hall. 324, 140, 144, 293-294
Schmidt D. and Livi K. J. T. (1999) – HRTEM and SAED investigations of polytypism, stacking disorder, crystal growth, and vacancies in chlorites from subgreeenschist facies out crops. Am. Miner., 84: 160–170

Chlorite Atomic Structure. Digital image. U. S. Geological Survey Open-File Report 01-041. Chlorite Atomic Structure. Digital image. U. S. Geological Survey Open-File Report 01-041

Oxide Weight%
Sample Name SiO2 Al2¬O3 TiO2 Fe203 Cr2O3 FeO MgO MnO CaO H2O
(-) H2O (+) Total
Sheridanite 27.30 24.17 Tr 1.87 - 5.15 29.24 Tr - .10 12.64 100.47
Clinochlore 30.60 16.80 - 2.18 - 5.02 32.18 - Tr 12.76 99.54
Penninite 33.78 13.24 0.00 1.50 - 3.07 34.41 0.16 0.00 13.89 100.05
Ripidolite 25.15 24.02 - 4.86 - 23.26 12.19 0.18 - 10.90 100.53
Brungsvigite 28.15 15.17 - 3.85 - 25.23 14.56 0.21 0.59 0.57 11.25 99.58
Diabantite 30.76 12.12 - 9.12 - 22.76 12.36 1.24 Tr 1.80 9.76 99.92
Thuringite 22.30 16.81 - 15.13 - 32.78 1.30 - - 11.04 99.36
Chamosite 26.56 20.19 0.24 1.27 - 36.51 2.68 2.04 0.24 0.40 9.97 100.10

Chlorite Group Minerals
Member Name Chemical Formula
Baileychlore (Zn,Fe,Al,Mg)6((Si,Al)4O10)(OH)8
Borocookeite Li1+x3 Al4-x (BSi3O10)(OH)8
Chamosite (Fe2+,Mg)5Al(AlSi3O10)(OH)8
Clinochlore (Mg,Fe2+)5Al(AlSi3O10)(OH)8
Cookeite (Al2Li)Al2(AlSi3O10)(OH)8
Corundophilite (Mg,Fe,Al)6(Si,Al)4O10(OH)8
Donbassite Al4.33(AlSi3O10)(OH)8
Franklinfurnaceite Ca2Fe3+Mn32+Mn3+(Zn2Si2O10)(OH)8
Nimite (Ni,Mg,Al)6((Si,Al)4O10)(OH)8
Orthochamosite (Fe2+,Mg,Fe3+)5Al(AlSi3O10)(OH,O)8
Pennantite Mn52+Al(AlSi3O10)(OH)8
Sudoite (Mg,Fe2+)2Al3(AlSi3O10)(OH)8

Private Mineral Project
Bonus Assignment Answer these questions about your private mineral:
1. What crystal system does it belong to? (This question translates into: what is the shape of the unit cell?)
Chlorite is mainly monoclinic, while triclinic or orthorhombic varieties are less common.
2. What are the unit cell dimensions? List values for edge lengths (a,b,c) and angles between the edges á, â, and ã. a = 5.37 Å , b = 9.30 Å , c = 14.25 Å , β = 97.4°, Z = 2.
3. What is the unit cell volume? (in cubic Angstroms)
711.66 Å
4. List the formula for your mineral. How many atoms are in the formula?
General formula is (Mg,Fe,Al)6 (Si,Al)4O10¬(OH)8 28 total atoms
5. How many atoms are in one unit cell? (This is equal to the number of atoms in the formula multiplied by the multiplicity, which is listed as a value for Z in reference books. It is generally a relatively small integer.)
56
6. What is the molar weight of your mineral?
739.05 g/mol -554.22 g/ mol
7. What is the weight of one formula unit of your mineral? (This equal to the molar weight divided by Avogadro’s number.)
1.227e-21 g– 9.202e-24 g 8. What is the weight of one unit cell? (This is equal to your answer to #7, above, multiplied by Z.)
2.454e-21 to 1.84e-21
9. What is the density of your mineral? (This is equal to the unit cell weight divided by the unit cell volume. Do the calculation – do NOT look up a value in a book because it is probably for a natural sample and, therefore, wrong for a pure sample.)
3.44e-14 g/m3- 2.59e-14 g/m3
10. What is the size (volume) of 1 mole of your mineral? Calculate this in cm3 and then compare the result to common objects such as a BB, a marble, a golf ball, a baseball, a softball, a basketball, etc. (The idea is to get some sense of size, not just a number.)
2.54e-11 m to 1.435e-11 m this is extremely small.